In conventional long term optical memory, data is recorded in localized data areas of fused silica glass modified by femtosecond laser pulses. At least some optical memory uses disc having multiple layers of microscopic data areas (dots) with differing refractive indices. In some instances, the microscopic data areas contain nanogratings, which include lamina structures embedded within the fused silica material. The nanogratings allow denser data writing due to each nanograting being characterized by individual orientation and retardance, which are controlled by polarization and intensity of writing laser beam. However, the size of data areas in one example is 3.7 μm, which limits the data density. For example, current compact discs have sub-micrometer data areas. Many known memory discs of fused silica glass with imbedded structures are produced by an expensive femtosecond laser technique, have low data density, and require complicated microscope systems to compensate for spherical aberrations and to extract data from multi-layer arrays of dots.
Optical memory discs are known that use a sapphire substrate and crystalline silicon film with localized data areas of silicon in amorphous phase formed by laser pulses. The principle of data storage for these discs is based on the difference in light transmission and reflectance for amorphous and crystalline silicon phases. The phase change of silicon is restorable using one laser pulse to transform crystalline phase of silicon into an amorphous phase in the data area, whereas the other laser pulse with different power and pulse duration may provide back transformation of the amorphous phase of silicon into the crystalline phase in the same data area, thus resulting in writing and erasing the data. However, the difference in light transmission and reflectance between amorphous and crystalline phases in silicon layers does not exceed about 10 times and the size of localized data areas is rather large at about 10 μm, which may limit the performance of the optical memory based on local phase change of silicon film.